Many processes and catalysts are known for the preparation of homopolymeric or copolymeric olefins and other polymers. Ziegler-Natta catalyst compositions, developed in the 1950s, were found to be particularly useful in the preparation of polyolefins. These catalyst compositions comprise transition metal compounds such as titanium tetrachloride and an alkylaluminum (e.g., triethylaluminum) cocatalyst. The systems were found to be advantageous because of their high activity, and were largely consumed during polymerization.
Subsequent catalyst systems have been designed to provide more control over polymer structure and properties than could be achieved with Ziegler-Natta catalysts. These later catalysts have well-defined active sites and can be rationally designed to produce a specific polymer product, i.e., having predetermined structure and properties. Such catalysts include, for example, metal complexes known as “metallocenes.” The term “metallocene” was initially coined in the early 1950s to refer to dicyclopentadienyliron, or “ferrocene,” a structure in which an iron atom is contained between and associated with two parallel cyclopentadienyl groups. The term is now used to refer generally to organometallic complexes in which a metal atom (not necessarily iron) is coordinated to at least one cyclopentadienyl ring ligand. A. D. Horton, “Metallocene Catalysis: Polymers by Design,” Trends Polym. Sci. 2(5):158-166 (1994), provides an overview of metallocene catalysts and their advantages, and focuses on now-conventional complexes of Group IV transition metal complexes and cyclopentadienyl ligands (Cp2MX2, wherein Cp represents a cyclopentadienyl ligand, M is Zr, Hf or Ti, and X is Cl or CH3). Unfortunately, however, although metallocenes do provide significant advantages relative to the traditional Ziegler-Natta catalysts, the high cost and difficulties associated with heterogenization of metallocenes, as well as the oxophilic nature of the early transition metals, have limited the applicability of metallocenes as commercial polymerization catalysts.
Because polyolefins such as polyethylene and polypropylene are such important commercial polymers, there is an ongoing need for improved polymerization techniques and polymerization catalysts. Recently, researchers have developed new catalysts suitable for olefin polymerization that are complexes of late transition metals and substituted diimine ligands. Such catalysts are described, for example, in Bres et al., PCT Publication No. WO 98/49208, published Nov. 5, 1998. Other similar catalysts, comprised of diimine ligands and selected metals, are described in Bennett, PCT Publication No. WO 98/27174, published Jun. 25, 1998, and in Brookhart et al., PCT Publication No. WO 98/30612, published Jul. 16, 1998.
A variety of catalyst systems have been used in the creation of “multimodal” polymer compositions, i.e., compositions containing two or more molecular weight distributions as may be determined, for example, by the appearance of two or more peaks in a gel permeation chromatogram or the like. The term “multimodality” can also refer to other characteristics of a polymer composition as well, e.g., compositional distribution (the distribution of comonomers within a copolymer), tacticity distribution (wherein a polymer composition contains at least two segments of differing tacticity, long-chain branching distribution, and the like. Such multimodal polymers are frequently more useful than compositions that are not; for example, multimodal polymer compositions can have improved rheological behavior, higher mechanical strength and increased elasticity relative to corresponding compositions that are not multimodal.
Several processes are known for preparing multimodal polymer compositions using the catalyst systems discussed above. In U.S. Pat. No. 5,032,562 to Lo et al., a process involving the use of tandem reactors operated in series is described wherein, in a first reactor, an olefinic monomer is catalytically polymerized in the presence of hydrogen, with the product then transferred to a second reactor in which polymerization is conducted in the presence of hydrogen. In this way, a higher molecular weight polymer is produced in the first reactor, and the lower molecular weight polymer is produced in the second reactor.
U.S. Pat. No. 5,525,678 to Mink et al. provides a supported catalyst composition for producing a polyolefin resin having a high molecular weight component and a low molecular weight component, wherein the catalyst composition contains a first catalyst that is a metallocene and a second catalyst that is a non-metallocene. The ratio of the high molecular weight and low molecular weight components in the polymeric product is determined by the ratio of the concentration of the two metals in the two-component catalyst composition. In addition, U.S. Pat. No. 4,659,685 to Coleman, III et al. Describes a two-component catalyst composition for preparing polyolefins having a molecular weight distribution which is multimodal, the catalyst composition comprising a mixture of a supported titanium compound and a separately supported or non-supported organometallic compound.
U.S. Pat. No. 5,032,562 to Lo et al., cited above, also relates to a supported olefin polymerization catalyst composition for producing high density polyethylene (“HDPE”) having a multimodal molecular weight distribution. The catalyst composition comprises: (1) a catalyst precursor supported on a porous carrier, and (2) a catalyst activator in the form of a mixture of conventional Ziegler-Natta cocatalysts. Katayama et al., “The Effect of Aluminum Compounds in the Copolymerization of Ethylene/α-Olefins,” in Macromol. Symp. 97:109-118 (1995), describes a similar system for preparing a polymer composition having a bimodal composition using a two-component catalyst comprised of a metallocene (Cp2ZrCl2) and either [Ph3C+][B(C6F5)4−] or [PhMe2NH+][B(C6F5)4−].
PCT Publication No. WO 92/00333, inventors Canich et al., and EP 416,815, inventors Stevens et al., are also of interest insofar as the references describe metallocene catalysts for preparing polyolefins. Canich et al. describes metallocene catalyst compositions for producing high molecular weight polyolefins having a relatively narrow molecular weight distribution, wherein the catalyst composition is comprised of (1) a metallocene containing a Group IVB transition metal coordinated to a cyclopentadienyl ligand, and (2) a coordination complex such as an anionic complex containing a plurality of boron atoms, which serves as a catalyst activator. The metallocene catalysts described may be mononuclear or binuclear (i.e., containing one or two metal atoms which serve as the active sites); the binuclear compounds dissociate during polymerization. Stevens et al. also pertains to metallocene catalysts to prepare addition polymers, particularly homopolymers and copolymers of olefins, diolefins, “hindered” aliphatic vinyl monomers and vinylidene aromatic monomers. The Stevens et al. catalysts are metal coordination complexes having constrained geometry, and are used in conjunction with a cocatalyst compound to form a complete catalytic system. The constrained geometry of the catalysts is stated to be of key importance insofar as the metal atom in the metallocene presumably is a more “exposed” active site.
Thus, the art provides metallocene catalyst compositions for producing polymers, particular polyolefins, that have a multimodal molecular weight distribution. However, such prior catalysts and catalyst compositions either require two or more components, e.g., two catalysts used in combination, or involve binuclear compounds that break apart into two separate components during the polymerization process (as in the bimetallic catalyst disclosed by Canich et al.), giving rise to potential manufacturing problems, e.g., phase separation or the like, and/or loss of control over the molecular weight distribution of the polymer composition prepared. In addition, the known metallocene catalysts can be relatively difficult and time-consuming to synthesize, requiring expensive equipment, extreme reaction conditions, and multi-step processes that ultimately result in a low yield of the desired product.
Accordingly, there is a need in the art for a simpler way of catalytically preparing multimodal polymer compositions while avoiding the high cost and difficulties associated with prior processes. An ideal method for preparing a multimodal polymeric product would involve a single catalyst that does not require the presence of a second catalyst, that retains its structure during the polymerization process, and is relatively simple to synthesize. The present invention is directed to such a catalyst.
The novel catalyst is comprised of an organometallic complex having two or more different active sites, at least one of which is composed of a transition metal atom coordinated to an unsaturated nitrogenous compound such as an imine, diimine or a 2,2′-bipyridine-containing compound. Use of such catalysts provide numerous advantages relative to the multimodal polymerization catalyst of the prior art, in that they:                1) allow for a exceptional control over the final polymer composition;        2) produce uniform multimodal products;        3) can be constructed via a straightforward, low cost synthesis;        4) are highly active polymerization catalysts;        5) can be used to catalyze reactions other than polymerization reactions, e.g., hydrogenation        6) enable preparation of commodity polymers such as linear low density polyethylene and isotactic polypropylene;        7) can be used as either supported or homogeneous polymerization catalysts;        8) are quite versatile and can be used in conjunction with a variety of monomer types; and        9) provide for all of the advantages typically associated with metallocene catalysts, i.e., versatility and use in conjunction with a variety of monomer types, the ability to control the degree of vinyl unsaturation in the polymeric product, and the like.        
The invention thus represents a significant advance in the field of catalysis, as prior to the development of the catalysts disclosed and claimed herein, only a few of the aforementioned advantages could be achieved with a single catalyst system.